The potent graft versus leukemia effect (GVL) associated with allogeneic bone marrow transplant (BMT) can produce lasting remissions in patients with myeloid leukemia, but the potentially lethal complication of graft versus host disease (GVHD) limits the effectiveness of this treatment. Donor T cells mediate both GVL and GVHD, although the target antigens recognized by these T cells are not known. We hypothesize that GVL would be enhanced and GVHD reduced or eliminated if the target antigens that drove those responses were identified and if T cells with GVL antigen specificity could be isolated and adoptively transferred to leukemia patients. We identified the first human leukemia-associated T cell antigen as PR1, an HLA-A2 restricted nonomer peptide derived from proteinase 3, an aberrantly expressed myeloid-restricted protein in leukemia cells. PR1/HLA-A2 tetramers were used to identify PR1-specific cytotoxic T lymphocytes (CTL) in chronic myeloid leukemia (CML) patients in cytogenetic remission after either interferon or BMT treatment. Using the same deductive strategy that identified PR1, we have shown that CTL with specificity for another antigen, MY4, an HLA-A2 restricted nonomer peptide derived from myeloperoxidase, comprise up to 3% of all CTL in acute myeloid leukemia (AML) patients that are in remission after nonmyelcablative stem cell transplant (NST) but are not detectable in patients that relapse. Furthermore, MY4-specific CTL, like PR1-specific CTL, selectively kill AML cells but not healthy bone marrow cells or epithelial cells, a target of GVHD. In this proposal, we will (1) apply this deductive strategy to uncover additional CTL leukemia-associated antigens (LAA), (2) determine whether LAA-specific CTL are present in patients that receive NST and (3) use LAA peptide/MHC tetramers to select and expand CTL for adoptive transfer into recipients of NST to enhance GVL and reduce GVHD.